[FEATURE REQUEST] Documentation for metabolic reaction network toolkit

[meren]

The need

For the new toys in our codebase made available through the heroic efforts of @semiller10 via #2123, we need,

• More comprehensive documentation for the new programs and artifacts
• Component tests with a minimal test scenario to keep them in our minds

The solution

Addressing the following concerns would be the best solution.

Docs

Here are some of the artifacts and programs that need love:

• [x] anvi-setup-modelseed-database (anvio/docs/programs/anvi-setup-modelseed-database.md, currently empty)
• [x] anvi-reaction-network (anvio/docs/programs/anvi-reaction-network.md, currently empty)
• [x] anvi-get-metabolic-model-file (anvio/docs/programs/anvi-get-metabolic-model-file.md, currently empty)
• [x] reaction-network (anvio/docs/artifacts/reaction-network.md, has minimal info)
• [x] reaction-network-json (anvio/docs/artifacts/reaction-network-json.md, has minimal info -- could use a brief example and more information regarding which flux balance analysis tools / standards it is compatible with)

Component tests

• [x] A minimal test case is missing for the new programs and their use cases -- anvio/tests/run_component_tests_for_metabolism.sh may be a good place to implement them.

Beneficiaries

Anvi'o users who are interested in understanding how to use these tools effectively :)

[semiller10]

Some of the documentation was stored in files without the requisite .md extension, but I fleshed it out a lot more with the help of your handy checklist.

I added reaction network tests to the metabolism self-test suite.

[meren]

You are the absolute best, Sam. Thank you very much.